apps/http_client: Fix speling issues

Found by codespell

src/apps/http/http_client.c

@@ -633,7 +633,7 @@ httpc_init_connection_addr(httpc_state_t **connection, const httpc_connection_t
  * @param settings connection settings (callbacks, proxy, etc.)
  * @param recv_fn the http body (not the headers) are passed to this callback
  * @param callback_arg argument passed to all the callbacks
- * @param connection retreives the connection handle (to match in callbacks)
+ * @param connection retrieves the connection handle (to match in callbacks)
  * @return ERR_OK if starting the request succeeds (callback_fn will be called later)
  *         or an error code
  */
@@ -678,7 +678,7 @@ httpc_get_file(const ip_addr_t* server_addr, u16_t port, const char* uri, const
  * @param settings connection settings (callbacks, proxy, etc.)
  * @param recv_fn the http body (not the headers) are passed to this callback
  * @param callback_arg argument passed to all the callbacks
- * @param connection retreives the connection handle (to match in callbacks)
+ * @param connection retrieves the connection handle (to match in callbacks)
  * @return ERR_OK if starting the request succeeds (callback_fn will be called later)
  *         or an error code
  */
@@ -819,7 +819,7 @@ httpc_fs_tcp_recv(void *arg, struct altcp_pcb *pcb, struct pbuf *p, err_t err)
  * @param uri uri to get from the server, remember leading "/"!
  * @param settings connection settings (callbacks, proxy, etc.)
  * @param callback_arg argument passed to all the callbacks
- * @param connection retreives the connection handle (to match in callbacks)
+ * @param connection retrieves the connection handle (to match in callbacks)
  * @return ERR_OK if starting the request succeeds (callback_fn will be called later)
  *         or an error code
  */
@@ -871,7 +871,7 @@ httpc_get_file_to_disk(const ip_addr_t* server_addr, u16_t port, const char* uri
  * @param uri uri to get from the server, remember leading "/"!
  * @param settings connection settings (callbacks, proxy, etc.)
  * @param callback_arg argument passed to all the callbacks
- * @param connection retreives the connection handle (to match in callbacks)
+ * @param connection retrieves the connection handle (to match in callbacks)
  * @return ERR_OK if starting the request succeeds (callback_fn will be called later)
  *         or an error code
  */

src/include/lwip/apps/http_client.h

@@ -53,7 +53,7 @@ extern "C" {
 
 /**
  * @ingroup httpc
- * HTTPC_HAVE_FILE_IO: define this to 1 to have functions dowloading directly
+ * HTTPC_HAVE_FILE_IO: define this to 1 to have functions downloading directly
  * to disk via fopen/fwrite.
  * These functions are example implementations of the interface only.
  */
@@ -116,7 +116,7 @@ typedef void (*httpc_result_fn)(void *arg, httpc_result_t httpc_result, u32_t rx
  * @param connection http client connection
  * @param arg argument specified when initiating the request
  * @param hdr header pbuf(s) (may contain data also)
- * @param hdr_len length of the heders in 'hdr'
+ * @param hdr_len length of the headers in 'hdr'
  * @param content_len content length as received in the headers (-1 if not received)
  * @return if != ERR_OK is returned, the connection is aborted
  */